feat(agent): add llm assistant chat with api tools (task 88)

OpenAI-compatible client with mock fallback, tool-calling loop, and /assistant UI.

Tools map to /api/v1/ endpoints per ADR 0006 direct-tools decision.

src/modules/index.ts

@@ -6,6 +6,7 @@ import notesManifest from "./notes/manifest";
 import gardenManifest from "./garden/manifest";
 import bangsManifest from "./bangs/manifest";
 import journalManifest from "./journal/manifest";
+import agentManifest from "./agent/manifest";
 
 registerModule(coreManifest);
 registerModule(calendarManifest);
@@ -14,3 +15,4 @@ registerModule(notesManifest);
 registerModule(gardenManifest);
 registerModule(bangsManifest);
 registerModule(journalManifest);
+registerModule(agentManifest);